For the purposes of the present invention, nitrile rubbers, also referred to as “NBRs” for short, are rubbers which are copolymers or terpolymers of at least one α,β-unsaturated nitrile, at least one conjugated diene and optionally one or more further copolymerizable monomers.
Such nitrile rubbers and processes for producing such nitrile rubbers are known, see, for example, W. Hofmann, Rubber Chem. Technol. 36 (1963) 1 and Ullmann's Encyclopedia of Industrial Chemistry, VCH Verlagsgesellschaft, Weinheim, 1993, pp. 255-261. This publication gives no indication as to whether and if appropriate how the vulcanization rate of such rubbers and the property profile, in particular the value of the modulus, can be influenced.
NBR is produced by emulsion polymerization, which firstly gives an NBR latex. The NBR solid is isolated from this latex by coagulation. Salts and acids are used for coagulation. In the coagulation of latices by means of metal salts, it is known that significantly larger amounts of electrolyte are required in the case of monovalent metal ions, e.g. in the form of sodium chloride, than in the case of polyvalent metal ions, e.g. in the form of calcium chloride, magnesium chloride or aluminium sulphate (Kolloid-Z. 154, 154 (1957)). It is also known that the use of polyvalent metal ions leads to “at least some inclusion of the emulsifier in the product” (Houben-Weyl (1961), Methoden der Org. Chemie, Makromolekulare Stoffe 1, p. 484). According to Houben-Weyl (1961), Methoden der Org. Chemie, Makromolekulare Stoffe 1, p. 479, “not only do the electrolytes used have to be very carefully washed out again, but the finished product should also be free of the catalysts and emulsifiers of the process batch. Even small amounts of residual electrolytes give turbid and cloudy pressed and injection-moulded parts, impair the electrical properties and increase the water absorption capacity of the finished product” (citation). Houben-Weyl gives no indication as to how a latex has to be worked up in order to give nitrile rubbers which vulcanize quickly and display a high modulus after vulcanization.
DD 154 702 discloses a process for the free-radical copolymerization of butadiene and acrylonitrile in emulsion, which is controlled by means of a specific, advantageously computer-aided metering program for the monomers and the molecular weight regulators, e.g. tert-dodecyl mercaptan, and in which the latices obtained are worked up by coagulation in an acid medium to give the solid rubber. A significant advantage of the process is said to be that the resin soaps and/or fatty acid soaps used as emulsifiers remain in the rubber as a result of the use of acids in the coagulation, i.e. they are not washed out as in the case of other processes. In addition to the advantage of good properties of the NBR, the improvement in the economics of the process and the avoidance of wastewater pollution by washed-out emulsifier are specifically advertised here. It is stated that the butadiene-acrylonitrile copolymers containing 10-30% by weight of acrylonitrile obtained have good elasticity and low-temperature properties combined with an increased swelling resistance and advantageous processability. Measures by means of which the vulcanization rate of the nitrile rubber and the property profile of the vulcanized NBR can be influenced are not revealed by the teachings of this patent.
JP 2790273 (Appl. 69 32,322) discloses that the use of amines in the coagulation of latices by means of magnesium salts, for example by means of a combination of diethylenetriamine and magnesium chloride, enables the initial vulcanization rate to be reduced and thus the scorch resistance of nitrile rubbers to be improved. Further information on this subject is not to be found in this prior art.
DE-A 23 32 096 discloses that rubbers can be precipitated from their aqueous dispersions by means of methylcellulose and a water-soluble alkali metal, alkaline earth metal, aluminium or zinc salt. Preference is given to using sodium chloride as water-soluble salt. It is stated that an advantage of this process is that it gives a coagulum which is virtually completely free of extraneous constituents such as emulsifiers, catalysts residues and the like since these extraneous materials are removed together with the water when the coagulum is separated off and any remaining residues are completely washed out by means of further water. Information about the vulcanization behaviour of rubbers produced in this way is not given. In DE-A 24 25 441, the electrolyte coagulation of rubber latices is carried out using 0.1-10% by weight (based on the rubber) of water-soluble C2-C4 alkylcelluloses or hydroxyalkylcelluloses in combination with from 0.02 to 10% by weight (based on the rubber) of a water-soluble alkali metal, alkaline earth metal, aluminium or zinc salt as auxiliary instead of methylcellulose. Here too, preference is given to using sodium chloride as water-soluble salt. The coagulum is separated off mechanically, optionally washed with water and the remaining water is removed. Here too, it is stated that the extraneous materials are, as in DE-A 23 32 096, essentially completely removed together with the water when the coagulum is separated off and any remaining residues are washed out completely in the washing with further water.
In DE-A 27 51 786, it is established that the precipitation and isolation of rubbers from their aqueous dispersions can be carried out by means of a smaller amount of (hydroxy)alkylcellulose when from 0.02 to 0.25% by weight of a water-soluble calcium salt is used. A further advantage is said to be that this process gives an extremely pure coagulum which is essentially completely free of extraneous constituents such as emulsifiers, catalysts residues and the like. These extraneous materials are removed together with the water when the coagulum is separated off and any remaining residues can be washed out by means of water. It is also stated that the properties of the isolated rubbers are not adversely affected by a calcium salt being used for coagulation. Rather, it is said that a rubber whose vulcanization properties are not impaired and are fully satisfactory is obtained. This is presented as surprising since it is said that impairment of the rubber properties is frequently observed when polymers are precipitated from dispersions by means of polyvalent metal ions such as calcium or aluminium ions. Houben-Weyl (1961), Methoden der Org. Chemie., Makromolekulare Stoffe 1, pp. 484/485, is offered as evidence for the last statement. In contrast, the rubbers of DE-A 27 51 786 display no slowing or worsening of, for example, the initial vulcanization and/or full vulcanization.
None of the documents DE-A 23 32 096, DE-A 24 25 441 and DE-A 27 51 786 disclose which measures have to be taken in order to achieve rapid vulcanization and good vulcanizate properties.
As in the case of the above-described patents, the object of DE-A 30 43 688, is also to achieve a large reduction in the amounts of electrolyte required for coagulation of the latex. According to the teachings of DE-A 30 43 688, this is achieved by using either plant-based protein-like materials or polysaccharides such as starch and if appropriate water-soluble polyamine compounds as auxiliaries in addition to the inorganic coagulate in the electrolyte coagulation of latices. As inorganic coagulates, preference is given to alkali metal or alkaline earth metal salts. The specific additives make it possible to achieve a reduction in the amounts of salts used for quantitative coagulation of the latex. DE-A 3 043 688 gives no information as to how rapid vulcanization can be achieved as a result of the production and/or work-up of the nitrile rubber.
In U.S. Pat. No. 4,920,176, it is stated and evidenced by experimental data that very high sodium, potassium and calcium contents and also emulsifiers remain in the nitrile rubber in coagulation of a nitrile rubber latex by means of inorganic salts such as sodium chloride or calcium chloride. However, this is undesirable and, according to the teachings of U.S. Pat. No. 4,920,176, water-soluble cationic polymers are used instead of inorganic salts in the coagulation of nitrile rubber latices for the purpose of obtaining very pure nitrile rubber. The polymers used here are, for example, ones based on epichlorohydrin and dimethylamine. These auxiliaries are used with the aim of significantly reducing the amounts of salts remaining in the product. The vulcanizates obtained therefrom display lower swelling on storage in water and an increased electrical resistance. In the patent text, the property improvements mentioned are attributed purely qualitatively to the minimal cation contents remaining in the product. A more detailed explanation of the phenomena observed is not given. U.S. Pat. No. 4,920,176 also gives no information as to whether and how the vulcanization behaviour and the magnitude of the modulus can be controlled by means of the production and work-up of the nitrile rubber.
The objective of EP-A-1 369 436 is to provide nitrile rubbers having a high purity. In particular, the residue emulsifier contents of these nitrile rubbers are very low. The particular cation contents in the form of the sodium, potassium, magnesium and calcium contents are also very low. The nitrile rubbers are produced by carrying out the emulsion polymerization in the presence of fatty acid and/or resin acid salts as emulsifiers, then carrying out coagulation of the latex by means of acids, optionally with addition of precipitants. As acids, it is possible to use all mineral and organic acids which allow the desired pH values to be set. As additional precipitant, use is made of, for example, alkali metal salts of inorganic acids. The fatty and resin acids formed here are subsequently washed out by means of aqueous alkali metal hydroxide solutions and the polymer is finally subjected to shear until a residual moisture content of less than 20% is obtained. EP-A-1 369 436 gives no information on the production of nitrile rubbers which display rapid vulcanization and a high modulus after vulcanization.
EP-A-0 692 496, EP-A-0 779 301 and EP-A-0 779 300 in each case describe nitrile rubbers based on an unsaturated nitrile and a conjugated diene. All the nitrile rubbers contain 10-60% by weight of unsaturated nitrile and have a Mooney viscosity in the range 15-150 or, according to EP-A-0 692 496, in the range 15-65 and all have at least 0.03 mol of C12-C16-alkylthio group per 100 mol of monomer units, with this alkylthio group having at least three tertiary carbon atoms and a sulphur atom which is bound directly to at least one of the tertiary carbon atoms.
The nitrile rubbers are in each case produced in the presence of a C12-C16-alkyl thiol having a corresponding structure as molecular weight regulator which functions as “chain transfer agent” and is thus incorporated as end group into the polymer chains.
In the case of the nitrile rubbers of EP-A-0 779 300, it is stated that they have a width “ΔAN” (AN=acrylonitrile) of the composition distribution of the unsaturated nitrile in the copolymer in the range from 3 to 20. The process for producing them differs from that of EP-A-0 692 496 in that only 30-80% by weight of the total amount of monomers is used at the beginning of the polymerization and the remaining amount of monomers is fed in only at a conversion of the polymerization of 20-70% by weight.
In the case of the nitrile rubbers of EP-A-0 779 301, it is stated that they contain 3-20% by weight of a fraction having a low molecular weight and a number average molecular weight M0 of less than 35 000. The process for producing them differs from that of EP-A-0 692 496 in that only 10-95% by weight of the alkyl thiol are mixed into the monomer mixture before the polymerization and the remaining amount of the alkyl thiol is fed in only after a polymerization conversion of 20-70% by weight has been reached.
With regard to the coagulation of the latex, all three patent applications EP-A-0 692 496, EP-A-0 779 301 and EP-A-0 779 300 state that any coagulants can be used. As inorganic coagulant, calcium chloride and aluminium chloride are mentioned and used. According to EP-A-0 779 301 and EP-A-0 779 300, a preferred embodiment is a nitrile rubber which is essentially halogen-free and is obtained by carrying out the coagulation of the latex in the presence of a non-ionic surface-active auxiliary and using halogen-free metal salts such as aluminium sulphate, magnesium sulphate and sodium sulphate. Coagulation using aluminium sulphate or magnesium sulphate is said to be preferred. The resulting, essentially halogen-free nitrile rubber has a halogen content of not more than 3 ppm.
In Comparative Example 6 of EP-A-779 300 and Comparative Example 7 of EP-A-0 779 301, the coagulation of the latex is carried out using a mixture of NaCl and CaCl2, with the CaCl2 being used in large amounts and the weight ratio of NaCl to CaCl2 being 1:0.75. In respect of the scorching time and the stress at 100% elongation, no significant differences from the other examples shown in the respective Table 12 or 13 are found.
According to EP-A-0 692 496, EP-A-0 779 300 and EP-A-0 779 301, it is essential to use alkyl thiols in the form of the compounds 2,2,4,6,6-pentamethylheptane-4-thiol and 2,2,4,6,6,8,8-heptamethylnonane-4-thiol as molecular weight regulators for the production of the nitrile rubbers. It is clearly pointed out here that the use of the conventional known tert-dodecyl mercaptan as regulator gives nitrile rubbers having poorer properties.

In the case of the nitrile rubbers produced in EP-A-0 692 496, EP-A-0 779 300 and EP-A-0 779 301, it is stated that they have an advantageous property profile, good processability of the rubber mixtures and make low fouling of the mould possible during processing. The vulcanizates obtained are said to have a good combination of low-temperature resistance and oil resistance and possess good mechanical properties. It is also stated that high polymerization conversions of greater than 75%, preferably greater than 80%, in the production of the nitrile rubbers enable a high productivity to be achieved and the vulcanization rate in vulcanization using sulphur or peroxides is high, in particular in the case of NBR grades for injection moulding. It is also indicated that the nitrile rubbers have a short initial vulcanization time and a high crosslinking density. As evidence of the rapid vulcanization of the nitrile rubbers produced according to EP-A-0 692 496, EP-A-0 779 300 and EP-A-0 779 301, the initial vulcanization time (known as the “scorch time” (measured as “T5”)) is presented, although this is merely a measure of the initial vulcanization rate. Nothing is said about the overall vulcanization rate and how this may be able to be influenced. The crosslinking density is described only by quotation of the maximum torque value (measured as Vmax).
In practice, short scorch times are not always desirable, since the corresponding rubber mixtures cannot be processed reliably because of such a fast initial vulcanization. Particularly in injection moulding, rapid initial vulcanization is not satisfactory. Short cycle times are critical for economical processing. To achieve short cycle times, the difference between full vulcanization rate and initial vulcanization rate is critical. This is measured as “t90-t10”, with t90 being the time at which 90% of the final vulcanization has taken place and t10 is the time at which 10% of the final vulcanization has taken place. However, use of the regulators 2,2,4,6,6-pentamethylheptane-4-thiol and 2,2,4,6,6,8,8-heptamethylnonane-4-thiol used in EP-A-0 692 496, EP-A-0 779 300 and EP-A-0 779 301 does not necessarily make setting of rapid vulcanization characteristics and setting of a high modulus possible.
On this subject, EP-A-0 692 496 indicates, inter alia, that many methods have already been proposed for setting high vulcanization rates, e.g. the use of minimal amounts of emulsifiers and precipitants, so that only minimal amounts of emulsifiers and precipitants remain in the NBR. However, according to EP-A-0 692 49, these measures are not satisfactory (p. 2, lines 22-28).
In summary, it may be said that, despite comprehensive literature, no measures which allow the overall vulcanization rate of nitrile rubbers and in particular the difference between full vulcanization rate and initial vulcanization rate (t90-t10) to be influenced without other important properties of the nitrile rubber, in particular the vulcanizate properties, being adversely affected have become known to the present time.